Density dependence doesn't wait for a crowding point
- The idea
- Predict
- See it
- Why
- Check
The idea
“Density only matters once a population gets crowded enough to cross some threshold.”
Crowding sounds like something that kicks in when a place gets packed. So a small, steady drop at low density looks like noise.
The short version
The mistake: Density only matters once a population gets crowded enough to cross some threshold.
What's actually true: A factor is density dependent when its effect per individual changes with density, even if the change is small and steady.
- A density-dependent factor, like competition for food, predation or disease, changes the birth or death rate per individual as density changes.
- In competition, each added individual takes a share of the food and space. So the effect can start with the first few and build steadily. No line has to be crossed.
- A density-independent factor, like a hard frost, removes about the same fraction at every density. A flat line is what no effect looks like.
On the exam: Ask if the per capita rate changes as density rises. A steady trend counts. Only a flat line means no effect.